Nitrite is produced from the breakdown of ammonia in the filter. Nitrite is highly toxic to fish, but is usually removed from the water by the bacteria which live inside the filter. However, in new tanks, or if the filter bacteria are damaged, the nitrite level may become toxic. The only way to know what the nitrite level is, is by testing it with a TetraTest Nitrite kit. Regular testing, especially in new aquariums, will prevent damage caused by nitrite poisoning.